The drugs -- Amgen Inc.'s Enbrel, Johnson & Johnson's Remicade and Abbott Laboratories Inc.'s Humira -- block a molecule called tumor necrosis factor that causes inflamed, painful joints. The TNF molecule also fights infection and possibly cancer.
But in documents released Monday, the FDA said a small number of patients have developed lymphoma while taking the drugs. The agency said the link isn't clear, however, because people with rheumatoid arthritis already have an elevated risk of developing lymph cancer.
The panel -- outside experts who regularly meet to assess the drugs' safety -- may request further study or recommend a warning on the arthritis drugs' labels.
Humira, the newest of the three drugs, already has a warning about lymphoma on its label.
"There is probably some pressure to treat the drugs equally," said Deutsch Bank Alex. Brown analyst Dennis Harp.
Harp said a lymphoma warning wouldn't hurt sales to patients with severe rheumatoid arthritis, which can cause joint damage so serious that people can wind up in wheelchairs. But a warning about cancer could put a dent in sales to people with moderate forms of rheumatoid arthritis, he said.
The drugs are sold to patients who haven't responded to the traditional mix of steroid medications and methotrexate, a chemotherapy drug. But Amgen, based in Thousand Oaks, is beginning to market Enbrel to the 1 million patients with moderate rheumatoid arthritis, a much larger market.
The three drugs already pose serious side effects, including an increased risk of infection. Humira and Remicade have warnings about a risk of tuberculosis, the strongest warning possible for prescription drugs.
Nonetheless, the drugs have been big sellers.
Remicade, also used to treat inflammatory bowel disease, had sales in excess of $1 billion in 2002. Production problems limited Amgen's Enbrel sales to $800 million in 2002, but the firm expects the drug's sales to reach $1.2 billion this year. Humira was approved for sale by the FDA in December.
In advance of the FDA meeting, Amgen said it proposed adding information about lymphoma to its label, but not in a warning. The company said its data show no additional risk to people who use Enbrel.
"Our rate of lymphoma is comparable to the general population," said Dawn Viveash, Amgen vice president of regulatory affairs and safety.
Tom Schaible, vice president of Centocor, the J&J; unit that sells Remicade, said: "We think the data is insufficient to show whether there is an association with anti-TNF drugs."
Analyst Harp said the FDA panel could recommend against a lymphoma warning if the evidence is weak.
On Monday, Amgen's stock slid 79 cents to close at $53.85 on Nasdaq. In New York Stock Exchange trading, Abbott fell 59 cents to $35.03, and J&J; dropped 2 cents to $52.43.